What is the source of CO2 and CH4 at 150 meters depth? Studying CO2, CH4 and N2O concentrations and carbon isotopic signatures in the air of the Vadose Zone in Spain
- 1University of Almería, Agronomy Department, 04120 Almería, Spain
- 2University of Almería, Biology and Geology Department, 04120 Almería, Spain
- 3University of Granada, Applied Physics Department, 18010 – Granada, Spain
- 4Andalusian Institute Earth System Research IISTA–CEAMA, 18006 Granada, Spain
- 5University of Granada, Ecology Department,18010 Granada, Spain
It is crucial to understand the cause-effect relationships of greenhouse gas (GHG: CO2, CH4 and N2O) concentrations and quantify their sources and sinks in natural systems, including their main reservoirs. This is particularly important when it comes to the vadose zone, which has the potential to store large amounts of GHGs in its pores. Currently, GHG measurements are mainly limited to the top few meters of soil, ignoring transport and storage processes in deeper areas. As a result, the vadose zone, with high concentrations of GHGs and a significant capacity for gas storage, is an enormous but unknown GHG reservoir.
To improve knowledge of GHGs in the vadose zone, the air column of boreholes, soils and the atmosphere were sampling over the course of a year, with three sampling campaigns, generating depth profiles of GHG concentrations down to 150 meters. This study shows the behavior of GHG concentrations found in the vadose zone of several sampling campaigns carried out in boreholes of different aquifers in the south of Spain. Also, we focus on two with results showing fluctuating concentrations of CO2 (419-8452 ppm), CH4 (0.56-63 ppm) and N2O (0.33-1504 ppm). The carbon isotopic signature for CO2 was -9 and -10 ‰ for the atmosphere, between -22 and -25 ‰ for the soil and between -10 and -22 depending of the air column of the borehole. The isotopic composition of CO2 from sectors closer to surface results from a mixing process between the soil-derived CO2 and the local atmosphere. The 13C enrichment of CO2 in some deeper sector of the air columns denotes the CO2 contribution by outgassing of water reservoirs within the vadose zone that has experienced a high interaction with the host-rock. The carbon isotopic signature for CH4 was especially surprising with values fluctuating between -9.76 and -100 ‰. The 13C enrichment of CH4 with concentrations above 50 ppm CH4 may indicate a particular case in which intense production coexists with a high consumption by methanotrophic bacteria and the 13C depletion of CH4 shows a biotic generation of methane. Although the complexity of these values may involve other processes that will be discussed.
These results reveal great variability in concentration and origin of the underground atmosphere, indicating the need for further investigation in order to accurately characterize this significant reservoir of greenhouse gases.
This work was supported by the OAPN through the project PN2021-2820s (IBERALP).
How to cite: Echeverría Martín, E., Fernández-Cortés, Á., Kowalski, A. S., Serrano-Ortiz, P., and Pérez Sánchez-Cañete, E.: What is the source of CO2 and CH4 at 150 meters depth? Studying CO2, CH4 and N2O concentrations and carbon isotopic signatures in the air of the Vadose Zone in Spain, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13572, https://doi.org/10.5194/egusphere-egu23-13572, 2023.
